1. Field of the Invention
The present invention relates to a magnetoresistive (MR) read transducer and, more particularly, to a magnetoresistive read transducer which provides a high amplitude readback signal with increased peak sharpness.
2. Description of the Related Art
A magnetoresistive (MR) read transducer (also a "read element") employs a magnetoresistive (MR) stripe or layer which changes resistance in response to magnetic fields. A sensing current, which is passed through the MR stripe, generates a signal voltage that varies proportionally to the change in resistance of the MR stripe. The useful response of the MR stripe (also called the "readback signal") is the signal voltage produced by the sense current and the resistance change of the MR stripe in response to the change in magnetic field, typically from a magnetic storage medium such as a rotating magnetic disk or a moving magnetic tape.
Typically, the MR stripe includes a thin film layer sandwiched between bottom and top insulation layers and which, in turn, are sandwiched between bottom and top shield layers. The distance between the shield layers is called "the read gap". The smaller the read gap, the greater the spatial resolution of the MR read transducer, permitting more faithful recovery of digitally recorded information and increased linear densities.
In some magnetic recording systems, digital information is stored in the form of the relative locations of successive transitions. Decoding this information is made more reliable by improving the ability to determine the relative positions of the peaks in the readback signal. Accordingly, it is important that the peak readback signal from the MR read transducer be as narrow as possible and have as high an amplitude as possible.
One of the problems with an MR read transducer is that the MR stripe can generate self-induced noise which is manifested in the readback signal from the magnetic medium. An MR stripe is typically made of a thin film of permalloy, which is an alloy of nickel and iron. This can have a multidomain magnetic structure. When a magnetic field is applied to the permalloy MR film, the walls of the magnetic domains within the MR stripe can move irregularly, producing what is classically known as Barkhausen noise. In order to overcome this noise problem, the prior art teaches providing a longitudinally oriented magnetic biasing field for the MR stripe. This greatly increases the probability that the MR stripe will be in the desired single-domain state. This can be accomplished by magnetostatic coupling between permanent magnetic material placed adjacent to the MR stripe or by exchange coupling between either a permanent magnet material or antiferromagnetic material and parts of the MR stripe adjacent to the active region.
Another problem with an MR read transducer is that its signal response is not linear with respect to the strength of the externally supplied magnetic field. This problem has been overcome in the prior art by transversely biasing the MR stripe. A typical approach is to magnetostatically couple a soft magnetic layer with the MR stripe, as described in U.S. Pat. No. 3,864,751. When a sense current flows through the MR stripe, a magnetic field is generated around the stripe which is induced into the soft film magnetic layer. The soft film magnetic layer becomes magnetized, which in turn induces a magnetic field back into the MR stripe to cause the transverse biasing. The combination of the longitudinal and transverse biasing develops a magnetization vector for the MR stripe which is at an angle to the longitudinal axis of the MR stripe or to the air-bearing surface (ABS) of the MR read transducer. This angle is typically chosen to be about 45.degree. to maximize the dynamic range of the MR stripe to an applied magnetic field.
One typical operating mode of an MR read transducer is to fly on an air cushion above a rotating magnetic disk. A suspension and a servo system operate to center the read transducer over a magnetically written track on the disk so that when the disk is rotated, a series of magnetic field signals, representing information, are imposed on the MR stripe. Since the design of an MR head normally follows the classic rule of "write wide and read narrow", a slight off-center positioning of the read element with respect to the track will not affect the response of the element.
A typical longitudinally and transversely biased MR transducer is described in U.S. Pat. No. 4,663,685. The MR read transducer, referred to as the continuous spacer exchange biased (CEB) transducer, employs an MR stripe located between a pair of passive portions. The passive portions are exchange-coupled to antiferromagnetic layers for the longitudinal biasing of the MR stripe. Upon the incursion of magnetic flux from a rotating disk, where the recorded tracks are wider than the central active portion of the transducer as described above, fields are also propagated in the passive portion of the CEB transducer. Prior to this patent, it was not recognized that these additional fields-from those parts of the track under the passive region--produce a supralinear cross talk signal which subtracts from the magnetic field produced by the parts of the track under the active portion of the MR stripe. As a consequence, this CEB transducer produces a readback signal which is wider and has a lower amplitude, and this is because the soft magnetic biasing layer has end portions that are magnetostatically coupled to the active portion of the MR stripe. In the presence of the wide-track magnetic field from the recording medium, the end portions of the soft magnetic material respond, and this in turn induces fields into the active portion of the MR stripe. Prior to this invention, it was not recognized that these fields are opposite in polarity to the field imposed on the active portion of the MR stripe.
Another prior art longitudinally and transversely biased MR read transducer is the contiguous-junction, hard-biased (CHB) transducer. In this transducer, a spacer is sandwiched between the MR stripe and a soft magnetic layer, and each of these layers is longitudinally biased at opposite ends by a pair of permanent magnets. The CHB transducer does not have any permeable layer portions which will add or subtract from the desired signal of the MR stripe.